Electrochemical storage batteries, and in particular, lead sulfuric acid storage batteries are ubiquitous in automotive applications. These batteries have electrochemical cells developing about 2.25 Volts each. A generic lead acid battery cell has a positive plate, a negative plate, and an electrolyte, typically aqueous sulfuric acid. The plates are held in a parallel orientation and electrically isolated by a porous separator to allow free movement of charged ions. Generally, six of these cells are connected in series to produce the 12 Volts (12 V) common in automobile systems.
The positive battery plate (also known as a positive electrode) contains a current collector (i.e., a metal plate or grid, hereinafter grid), covered with a layer of positive active material (hereinafter PAM) on the surface. PAM is essentially all electrically conductive lead dioxide (PbO2). The negative battery plate contains a current collector (grid), and it is covered with a negative active material, typically spongy lead.
Lead acid battery cells are quite unique because the electrolyte actively participates in the energy storage and release process, as represented schematically in Equations 1, 2, 3, and 4 below:                                           Electrolyte                                                              xe2x80x83                            ⁢                                                                    H                    2                                    ⁢                                      SO                    4                                                  ⇌                                                      H                    +                                    +                                      HSO                    4                    -                                                                                                          Equation        ⁢                  xe2x80x83                ⁢        1                                                                    Negative              ⁢                              xe2x80x83                            ⁢              Electrode                                                                          xe2x80x83                            ⁢                                                                    Pb                                          (                      metal                      )                                                        +                                      HSO                    4                    -                                                  ⁢                                  ⇌                  Charge                  Discharge                                ⁢                                                      PbSO                    4                                    +                                      H                    +                                    +                                      2                    ⁢                                          e                      -                                                                                                                              Equation        ⁢                  xe2x80x83                ⁢        2                                                                                    Positive                ⁢                                  xe2x80x83                                ⁢                Electrode                                                                                      xe2x80x83                                ⁢                                                                            PbO                      2                                        +                                          3                      ⁢                                              H                        +                                                              +                                          HSO                      4                      -                                        +                                          2                      ⁢                                              e                        -                                                                              ⁢                                      ⇌                    Charge                    Discharge                                    ⁢                                                            PbSO                      4                                        +                                          2                      ⁢                                              H                        2                                            ⁢                      O                                                                                                          _                            Equation        ⁢                  xe2x80x83                ⁢        3                                                                                    Total                ⁢                                  xe2x80x83                                ⁢                Reaction                            ⁢                              xe2x80x83                                                                                                          Pb                                      (                    metal                    )                                                  +                                  PbO                  2                                +                                  2                  ⁢                                      H                    2                                    ⁢                                      SO                    4                                                              ⁢                              ⇌                Charge                Discharge                            ⁢                                                2                  ⁢                                      PbSO                    4                                                  +                                  2                  ⁢                                      H                    2                                    ⁢                  O                                                                                        Equation        ⁢                  xe2x80x83                ⁢        4            
Discharge within the electrochemical cell results in lead metal (Pb) supplied by the negative plate reacting with the ionized sulfuric acid electrolyte to form lead sulfate (PbSO4) on the surface of the negative plate (see Equation 2). Discharge also results in the PbO2 located on the positive plate being converted into PbSO4 on or near the positive plate. Charging of the battery cell (via an electron supply from an external electrical current) converts PbSO4 into spongy lead metal on the surface of the negative plate, and converts PbSO4 into PbO2 (PAM), on the surface of the positive plate. In effect, charging converts PbSO4 into PAM and lead metal; discharging releases the stored electrical potential by converting PAM and lead metal back into PbSO4.
Accordingly, it is important to battery performance that the PAM be in physical and electrical contact with the positive plate. As such, the PAM must be supported by, adhered and/or attached to, and in electrical communication with the positive grid for the battery to function properly. Separation of PAM from the positive plate results in poor battery performance and ultimately in battery failure, which is defined herein as a battery no longer suitable for its intended purpose.
Factors that affect battery performance include the chemical make-up of the positive grid, the geometric configuration of the positive grid, and the method by which the grid is converted (i.e., processed) into a positive plate. Chemically, battery grids are lead containing alloys. The geometry (i.e., spatial arrangement) of a grid depends on the method by which the grid is made. Conversion of a grid into a positive battery plate involves a series of process steps. Historically, this process includes pasting, steaming, curing, pickling, and/or forming.
In pasting, water, sulfuric acid, lead and lead oxides, among other materials are applied to the grid surface as a paste. The pasted grid may then be steamed (e.g., 100xc2x0 C. and 100% humidity) to facilitate crystal growth within the paste. The pasted grid is then cured at controlled temperature and humidity conditions to xe2x80x9csetxe2x80x9d the paste, wherein the paste is chemically transformed into sulfates, hydroxides, and other lead compounds through a series of complex hydration reactions requiring the presence of water. These reactions take place within the paste itself, and between the paste and the grid to produce a xe2x80x9ccorrosion layerxe2x80x9d at the interface between the grid and the paste. This corrosion layer will subsequently provide both physical and electrical communication between the grid, and the later formed PAM.
Once cured, grids are assembled into a battery package and a charging electrolyte is added. The grids are xe2x80x9cpickledxe2x80x9d by allowing them to stand for a period of time in contact with the charging electrolyte. Next, the grids are xe2x80x9cformedxe2x80x9d by passing an external electric current through the cell. In forming, a majority, if not all of the paste on the positive plate is converted into PAM. The charging electrolyte is then replaced with shipping electrolyte to render the battery ready for use.
Phenomena that have a negative effect on battery performance include fractures in the PAM layer due to stress introduced as the layer accumulates on the positive plate during charging. The metal grid also reacts with the sulfuric acid electrolyte through pores, cracks, or holes in the corrosion layer to destroy the interface between the grid and the PAM. Such grid corrosion breaks both electrical communication and physical contact (i.e., support) between the grid and the layer of PAM.
Also, lead contained within the positive grid can be oxidized to form the corrosion layer of combined lead oxide (PbO) and lead dioxide on the surface of the grid according to Equation 5 below:
Pb(metal)+H2Oxe2x86x92PbOxe2x80x83xe2x80x83Equation 5 
PbO formation renders the corresponding portion of the grid surface non-conductive, and also negatively impacts support of the PAM layer. Accordingly, these phenomena, either alone or in combination, result in decreased battery performance (i.e., the cell losing its capacity to transfer and store electrical energy), eventually leading to battery failure. Furthermore, while these phenomena are significant at room temperature, they become even more significant at higher operational temperatures.
The rate at which a chemical process proceeds is proportional to temperature. In general, the higher the temperature, the faster the reaction rates within the lead acid battery cell (i.e., the higher the temperature, the more PAM that forms, the more PbO that can form, and the more corrosion of the grid that can take place). Positive grid corrosion becomes particularly significant under xe2x80x9chigh temperaturexe2x80x9d conditions (defined herein as above 50xc2x0 C.), which have become common in automotive applications as xe2x80x9cunder hood temperaturesxe2x80x9d rise due to automotive design trends and space limitations.
Another temperature driven phenomenon detrimental to battery performance is PAM separation that results from dissimilar thermal expansion coefficients between the PAM layer and the underlying grid. Typically, the grid expands faster than does the PAM, causing the PAM layer to crack, rupture and separate, which serves to exacerbate grid corrosion.
Accordingly, it is desirable to reduce or substantially eliminate high temperature effects on lead acid battery plates. In particular, to provide a longer useful-lifetime of the battery, preferably utilizing materials and processes that provide an economic incentive in doing so.
Provided for herein is a method of making an expanded metal grid, comprising: compression rolling a metal strip at a reduction ratio from about 1.25 to 1, to about 25 to 1 to produce a rolled strip, heating the rolled strip at a temperature of at least about 125xc2x0 C., and at most about 325xc2x0 C. for at least about 30 seconds, to produce a heat treated metal strip having an equiaxial grain structure within; and expanding the heat treated metal strip to produce the expanded metal grid.
Also provided for is a method of making an expanded metal grid, comprising: compression rolling a metal strip at a reduction ratio between about 1.25 to 1, and 25 to 1 to produce a rolled metal strip, expanding the rolled metal strip to produce an expanded metal grid; and heating the expanded metal grid at a temperature between about 125xc2x0 C., and about 325xc2x0 C. for at least about 30 seconds to produce an equiaxial grain structure within the, metal grid.